10-08-2013, 12:51 PM
Study of Magnetic Refrigeration
Introduction
Magnetic refrigeration is a cooling technology based on the magneto caloric effect. This
technique can be used to attain extremely low temperatures (well below 1 Kelvin), as well as the
ranges used in common refrigerators, depending on the design of the system. As Energy input to
the magnet is increased the orientation of the magnetic dipoles in a magnet starts loosing
orientation. And vice a versa at Currie temperature as magnet looses energy to the media it
regains the property.
The effect was discovered in pure iron in 1881 by E. Warburg. Originally, the cooling effect
varied between 0.5 to 2 K/T. Major advances first appeared in the late 1920s when cooling via
adiabatic demagnetization was independently proposed by two scientists: Debye (1926) and
Giauque (1927). The process was demonstrated a few years later when Giauque and MacDougall
in 1933 used it to reach a temperature of 0.25 K. Between 1933 and 1997, a number of advances
in utilization of the MCE for cooling occurred.This cooling technology was first demonstrated
experimentally by chemist Nobel Laureate William F. Giauque and his colleague Dr. D.P.
MacDougall in 1933 for cryogenic purposes (they reached 0.25 K).Between 1933 and 1997, a
number of advances occurred which have been described in some reviews. In 1997, the first near
room temperature proof of concept magnetic refrigerator was demonstrated by Prof. Karl A.
Gschneidner, Jr. by the Iowa State University at Ames Laboratory. This event attracted interest
from scientists and companies worldwide who started developing new kinds of room temperature
materials and magnetic refrigerator designs.
Magneto caloric Wheel:
It forms the structure of the whole device. It joins both the two
magnets to work properly.
The magnetic refrigeration is mainly based on magneto caloric effect according to which some
materials change in temperature when they are magnetized and demagnetized. Near the phase
transition of the magnetic materials, the adiabatic application of a magnetic field reduces the
magnetic entropy by ordering the magnetic moments. This results in a temperature increase of
the magnetic material. This phenomenon is practically reversible for some magnetic materials;
thus, adiabatic removal of the field reverts the magnetic entropy to its original state and cools the
material accordingly. This reversibility combined with the ability to create devices with inherent
work recovery, makes magnetic refrigeration a potentially more efficient process than gas
compression and expansion. The efficiency of magnetic refrigeration can be as much as 50%
greater than for conventional refrigerators.
Magnetic Materials
Only a limited number of magnetic materials possess a large enough magneto caloric effect to be
used in practical refrigeration systems. The search for the "best" materials is focused on rare-
earth metals, either in pure form or combined with other metals into alloys and
compounds(Fig.4.4). The magneto caloric effect is an intrinsic property of a magnetic solid. This
thermal response of a solid to the application or removal of magnetic fields is maximized when
the solid is near its magnetic ordering temperature.
The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly
dependent upon the magnetic order process: the magnitude is generally small in
antiferromagnets, ferrimagnets and spin glass systems; it can be substantial for normal
ferromagnets which undergo a second order magnetic transition; and it is generally the largest for
a ferromagnet which undergoes a first order magnetic transition.
Regenerators
Magnetic refrigeration requires excellent heat transfer to and from the solid magnetic material.
Efficient heat transfer requires the large surface areas offered by porous materials. When these
porous solids are used in refrigerators, they are referred to as "regenerators”. It is a heat
exchanger system which provide maximum area for efficient heat transfer. Material used for heat
exchanger should have magneto caloric property. By providing different arrangement for flow of
air over it the efficient heat transfer can be achieved. This regenerators can be classified
according to arrangement of magneto caloric materials.
Super Conducting Magnets
Most practical magnetic refrigerators are based on superconducting magnets operating at
cryogenic temperatures (i.e., at -269 C or 4 K).These devices are electromagnets that conduct
electricity with essentially no resistive losses. The superconducting wire most commonly used is
made of a Niobium-Titanium alloy Fig.4.6. Only superconducting magnets can provide
sufficiently strong magnetic fields for most refrigeration applications. A typical field strength is
8 Tesla (approximately 150,000 times the Earth's magnetic field).An 8 Tesla field can produce a
magneto caloric temperature change of up to 15 C in some rare-earth materials.
Advantages Over Vapour Compression Cycles:-
Magnetic refrigeration performs essentially the same task as traditional compression-cycle gas
refrigeration technology. Heat and cold are not different qualities; cold is merely the relative
absence of heat. In both technologies, cooling is the subtraction of heat from one place (the
interior of a home refrigerator is one commonplace example) and the dumping of that heat
another place (a home refrigerator releases its heat into the surrounding air). As more and more
heat is subtracted from this target, cooling occurs.
Conclusion
Magnetic refrigeration technology could provide a „green‟ alternative to traditional energy-
guzzling gas-compression fridges and air conditioners. They would require 20-30 percent less
energy to run than the best systems currently available, and would not rely on ozone-depleting
chemicals or greenhouse gases. Magnetic refrigeration is a technology that has proven to be
environmentally safe. Computer models have shown 25% efficiency improvement over vapor
compression systems. In order to make the Magnetic Refrigerator commercially viable, scientists
need to know how to achieve larger temperature swings. Two advantages to using Magnetic
Refrigeration over vapor compressed systems are no hazardous chemicals used and they can be
up to 60% efficient.